(1) Field of the Invention
The present invention generally relates a downhole compressor, and more specifically to a compressor designed to be lowered into a well of a natural gas reservoir to assist in extracting gas from the reservoir.
(2) Description of Related Art
It is known in the art that the gas flowing from a well drilled into a gas reservoir frequently carries with it a burden of vapor and liquid droplets. The pressure of gas at the base of a well falls as gas is extracted. Consequently the flow velocity of the gas in the production tubing also falls, and eventually becomes too low to carry its burden of condensed liquids. As a result, liquid accumulates at the base of the well, the gas flow falls and eventually ceases. Gas production ceases to be economically effective before the gas flow ceases and operators will normally abandon a well long before the gas supply is exhausted.
It has previously been proposed in the PCT patent application number WO97/33070 to install into the well an electrically or hydraulically powered gas compressor to rest at the bottom of the well. The effect of the compressor is to accelerate production and increase the ultimate recovery from the reservoir. In the first place, the compressor acts to reduce the static pressure at its inlet which increases the pressure difference between the reservoir and the well, so as to stimulate greater flow. Second, by increasing the gas pressure, the compressor increases the average density which leads to a reduction in flow velocity and hence in a reduction in the pressure losses along the length of the well. A further effect of the compression is to raise the temperature of the gas and thereby delay condensation of vapor.
Though the latter patent application discloses the concept of what is herein termed a downhole compressor, the compressor that it teaches has several limitations that would make it impracticable. For example, the electric motor used to drive the rotor shaft carrying the impellers that compress the produced gas is connected to the rotor shaft through gearing which allows the motor to rotate much more slowly than the impellers. This design is to enable the motor to be oil cooled and oil lubricated while air bearings are used to support the shaft carrying the impellers. However, this presents problems with the maintenance of the reduction gearing which are not addressed in the application. Furthermore, the application gives no details of how the gas bearings supporting the rotor shaft can be constructed or configured to receive an adequate supply of clean gas, nor does it resolve the rotor dynamic requirements of a shaft system supported on both gas and liquid lubricated bearings.
Accordingly, a need exists to provide a rotary compressor to overcome the above-mentioned limitations.